Cyclic allyl cations

The H and n.m.r. spectra of a variety of cyclic allylic cations have been reported cation stability decreases with increasing ring size, and parent cyclo-octenyl and cyclononenyl ions are too unstable to observe. Solvolysis of (321 n = 7 or 8) with aqueous ethanol and NEtj occurs more rapidly than... 

Unlike the cyclohexen-3-yl cation, which behaves like its acyclic counterparts, cyclic allyl cations have long been considered to be special cases. Solvolysis rates as well as stability... 

Ichikawa and co-workers have examined the Nazarov cyclization of difluorodienone 107 (Scheme 19.28) and have shown that it leads selectively to cross-conjugated dienone product 110. The Nazarov cyclization was initiated by exposure of 107 to trimethylsilyl triflate in a mixed solvent of dichloromethane and hexafluoroisopropanol (HFIP), leading to the resonance tautomers 108 and 109 of the intermediate cyclic allylic cation. Tautomer 109 is disfavored because of the electrostatic effect of the two fluorines on the adjacent carbon atom therefore, the process is terminated through proton loss from the methyl group rather than from the n-butyl group. Elimination of one of the (3 fluorine atoms leads to 110 as the observed final product. 

The case of a-fluorodienone 111 illustrates the well-known tendency of fluorine to stabilize an a cation (Scheme 19.29). In this case, exposure of 111 to Lewis acid leads to resonance tautomers 112 and 113 of the intermediate cyclic allylic cation. Tautomer 113 is favored by fluorine substitution on the positively charged carbon atom, biasing proton loss that leads to a-fluoroenone 114. Other approaches for the control of the regiochemistry of the double bond in Nazarov products have been published. 

Both cyclic and acyclic allylic cations have been produced in this way. Stable allylic cations have also been obtained by the reaction between alkyl halides, alcohols, or alkenes (by hydride extraction) and SbFs in SO2 or S02C1F. Divinylmethyl... 

Other factors which affect the case of electrocyclic ring opening include the nature of substituents which can stabilize or destabilize the development of possible charge and the release of strain in small cyclic systems. Thus different stereochemistries have been observed in the ring opening of cyclopropyl derivatives. All cis derivatives generate an all-cis allyl cation but the anti derivatives will form the all trans cation. 

Homoallyl rearrangements occur in the solvolyses of allenic /j-toluenesulfonates 1 (5-tosy-loxypcnta-1,2-dienes, ethenylidene p-toluenesulfonates). Hydrolysis or acetolysis of acyclic and cyclic compounds indicates that the allene group participates strongly to give allyl cations which rearrange to methylenecyclobutanols 2 (see Houben-Weyl, Vol. 4/4, p63). 

Both cyclic and acyclic allylic cations have been produced in this way. Stable allylic cations have also been obtained by the reaction between alkyl halides, alcohols, or olefins (by hydride extraction) and SbF5 in SO2 or SOjCIF.26 Divinylmethyl cations27 are more stable than the simple allylic type, and some of these have been prepared in concentrated sulfuric acid.2 Arenium ions (p. 502) are important examples of this type. Propargyl cations (RCssCCR2 ) have also been prepared.29... 

In the presence of a cationic Rh[((/ )-binap)(cod)] complex, geranyl or neryl amides isomerize slowly to give a mixture of the corresponding enamide and dienamide (Scheme 20) (2). The optical purity of the chiral enamide is high, but the chemical yield is low. Certain cyclic allylic amides give the enamide isomers in a high ee. With a DIOP-Rh catalyst, prochiral allylic alcohols are converted to optically active aldehydes with low ee (31). 

Reaction of the optically active allene (205) with bromine leads to the inactive 3-bromodihydropyran (208 X = Br), whereas activity is retained in the reaction with 2,4-dinitrobenzenesulfenyl chloride, giving (208 X = SC6H3(N02)2 Scheme 39) (67JA7001). The different behaviour has been attributed to the varying stabilities of the cyclic intermediates (206 X = Br or SC6H3(N02)2), the latter showing less tendency to rearrange to the achiral allylic cation (207). 

Roberts and coworkers, investigating the ionization reactions of cyclobutene derivatives, found that the resulting cyclobutenyl ions were unusually stable10-113. They suggested that rather than regarding these ions as simple allyl cations, their properties were consistent with a C(1 ),C(3) interaction and cyclic delocalization of the 7t-electrons. As such, these 27r-electron systems were considered to be the homoaromatic counterparts of the well established, aromatic cyclopropenium ions39. 

The intramolecular 4 + 3-, 3 + 3-, 4 + 2-, and 3 + 2-cycloaddition reactions of cyclic and acyclic allylic cations have been reviewed, together with methods for their generation by thermal and photochemical routes.109 The synthetic uses of cycloaddition reactions of oxyallyl cations, generated from polybromo and some other substrates, have also been summarized seven-membered rings result from 4 + 3-cycloadditions of these with dienes.110 The use of heteroatom-stabilized allylic cations in 4 + 3-cycloaddition reactions is also the subject of a new experimental study.111 The one-bond nucleophilicities (N values) of some monomethyl- and dimethyl-substituted buta-1,3-dienes have been estimated from the kinetics of their reactions with benzhydryl cations to form allylic species.112 Calculations on allyl cations have been used in a comparison of empirical force field and ab initio calculational methods.113... 

The observation of stable alkyl cations in antimony pentafluoride solutions also opened up the possibility of investigating the electronic spectra of these solutions. It has been reported73 that solutions of alkyl cations in HSC F-SbFs solution at — 60°C showed no absorption maxima above 210 nm. In view of this observation, it was resolved that previous claims relating to a 290-nm absorption of alcohols and alkenes in sulfuric acid solutions were due to condensation products or cyclic allylic ions and not to simple alkyl cations.74... 

This method is remarkable in that ketones can also be allylated with high levels of enantiocontrol, a transformation that is rarely encountered in the literature. Tietze postulated that the cyclic oxazolidinium cation 63 acted as the key intermediate in the asymmetry-inducing allylation step (Scheme 13.26). 

Allyl)CpFe(CO)(PR3) complexes (14) have been prepared from the dicarbonyls, by photolysis in the presence of phosphine or phosphite. The substitution is often aided by a trace of (CpFe(CO)2)2, which is indicative of a radical (see Radicals) chain substitution mechanism. Phosphite ligand (as in 15) has been reported as being a particularly good replacement ligand from the standpoint of thermal stability.Nevertheless, neither C-3-substituted ()] -allyl)Fp complexes (14b) nor cyclic allyl complexes (see Allyl Complexes) may be made directly by this method the carbocyclic cases have been prepared by methoxide-induced proton abstraction of aUcene cation (16) (Section 4.3.2). " ... 

The clathrochelate [CoPcc(BF)]+ cation has been used as a paramagmetic phosphine shift reagent for the elucidation of the structures of transition metal complexes (in particular, (substituted-allyl)-palladium ones. Scheme 126) in solution [430, 431]. The resonance in the NMR spectra of the complexes obtained was assigned and applied to isomers and cyclic allyls identification. The reproducibility and transferability of NMR chemical shifts as well the... 

Cyclic oxocarbenium cations approximate the a- and jS-chelation models for allylations of substituted aldehydes. In this analogy, the nature and the degree of substitution of the carbon bound to the oxonium site replaces the role of coordinated bidentate Lewis acids. Aside from the obvious concerns about the relative reactivity of 126 (Scheme 5.2.27), as compared with metal cation activation of precomplexed aldehyde 127, the C—O bond of 126 is much shorter (1.514 A) than distances associated with the... 

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